The present invention relates to a boiler element intended to be traversed, in use, at least partly by water to be heated and to form, together with other boiler elements placed alongside it, and coupled with and hydraulically connected to each other, a multi-fuel boiler for heating systems. Between the elements thus assembled, passages are delimited for the combustion or flue gases coming from the combustion chamber, into which passages fins project which are rigid with the boiler elements and arranged to transfer heat from the combustion gases to the water to be heated.